I am not a fan of the convergent evolution argument for humanoid aliens. I can well believe that it’s likely that intelligent aliens exist out there in the universe, but I’m not even going to try to predict what they look like: there are too many alternative paths that are possible. But for some reason, many people like to insist that it’s reasonably likely that they’d resemble us in general, if not in detail, and they’ll then go on to extrapolate that behaviorally and culturally they’ll share many properties with us. Usually, as with Simon Conway Morris and now George Dvorsky, this argument relies entirely on the premise of convergent evolution.
It annoys the hell out of me, because it requires waving away or consciously ignoring basic principles of evolution. Here’s how Dvorsky illustrates the concept:
I’ve seen versions of this illustration a thousand times. Gosh, look, dolphins and ichthyosaurs all have paddle-shaped fins and streamlined bodies! Therefore, this is evidence that aquatic forms all converge on similar morphologies. And therefore, intelligent, terrestrial organisms will also converge on an ideal form for their niche, which is ours also, therefore we represent a morphological ideal.
I hate the argument because it isn’t applicable to alien species. In both cases shown above, the organisms involved belong to the same subphylum, the Vertebrata, and the same superclass, the Tetrapoda. They share a common ancestor, and the same starting point. When you start with a terrestrial creature that looks like this, with four limbs and a suite of similar traits…
…and then let it re-adapt to a marine life as a free-swimming, active predator after long detours into mammalian and reptilian forms, is it any surprise that they converge on similar structures? They are both constrained by their ancestry! The four-finned torpedo does not necessarily represent an ideal form that will be recreated on every planet we ever someday visit, but is instead a compromise, a form that can be generated from a four-limbed vertebrate with a minimum of fuss.
But what if we look at organisms with more remote ancestry? A whole different phylum, perhaps? If we start down the evolutionary road with a conchiferan sort of creature, a mollusc…
…which later evolves into a free-swimming, active predator, you get this:
That’s something that looks completely different from a fish-like ichthysaur or dolphin; it’s got a completely different shape, a completely different set of feeding behaviors, a completely different internal organization.
We could ask the same question of other phyla. Where are the sleek torpedo shaped crustaceans sporting a nice dorsal fin and quartet of paddles? Show me a marine annelid that has followed this same path.
Now keep in mind that life on another planet will share no ancestry with anything on Earth. If our history is any example, they will be the product of a few billion years of single-celled tinkering, with a riotous adaptive radiation of multicellular forms that will explore a small fraction of morphospace…and every step will be contingent on prior states.
You can only make this ludicrous convergence argument if you think 1) contingency is relatively unimportant, that 2) adaptation is extremely powerful and will always drive a species towards an optimum, and that 3) the shape of a relatively tiny subset of species on this planet represent that optimum. There is a fourth requirement as well: you must be oblivious to the fact that (2) and (3) contradict each other.